Field of the Invention
The invention relates, in general, to a de-emphasis filter for SECAM-coded picture signals. Such filters are used in SECAM decoders for television or video equipment.
The SECAM Standard provides for the color information of an image to be transmitted with half as large a spatial resolution as the brightness information. In effect, color information is transmitted only with each second picture line. Since a complete item of color information includes two different types of color information in addition to the brightness, only one of the two items of information is modulated onto the signal of the relevant line, the other being transmitted with the line following thereupon. In order to be able to distinguish the two different items of color information at the receiver, two different carrier frequencies f.sub.r and f.sub.b are used for the two types of information. The lines thus modulated are denoted correspondingly as D.sub.r lines and D.sub.b lines, respectively.
The incoming color information signals are mixed in the receiver with a frequency of f.sub.0 =4.286 MHz. This yields base frequency signals with a constant offset that corresponds in each case to the difference between the respective color carrier frequency and the mixed frequency:
f.sub.r -f.sub.0 and f.sub.b -f.sub.0,
respectively. These frequencies must be very constant in order to avoid errors in the color rendition. The transmitter therefore transmits, in the burst gate period of the signal, identification pulses that are intended to permit the detection of a line as a D.sub.r line or D.sub.b line and to permit the adjustment of the mixed frequency f.sub.0.
A special circuit must be provided in each SECAM decoder for the purpose of evaluating these pulses and forming a mean value over a plurality of rows (the achromatic value).